Group calls have historically been an important feature in radio dispatch services such as police and rescue services. More recently, such features have been made available to communication units in wireless communication networks by some service providers. In addition to normal cellular network services, some service providers maintain an entirely separate wireless or cellular network having frequencies and equipment dedicated to specialized services. Some specialized networks are based on the Integrated Digital Enhanced Network developed by Motorola, Inc. of Schaumburg, Ill. and use, for example, the 800 MHz portion of the radio spectrum assigned to specialized mobile radio (SMR) service. In accordance with this network, for example, slotted TDMA technology is used to provide normal cell phone voice communications, messaging services such as pager and e-mail, data services such as wireless Internet and private data networks, digital two-way radio or dispatch services such as one-to-one and group communication. Group calls rely on a Push-To-Talk (PTT) call initiation commonly used in dispatch radio systems requiring a speaker in an active communication to press a talk button before talking and to release the talk button when finished talking. A listener can likewise press and release a talk button to respond. When making a group call in a PTT environment, a button can be pressed on a communication unit configured with the number or numbers of the target or targets for the group call to establish a call or session with, for example, the above noted network. The network determines that the call is a dispatch or group call rather than an interconnect call such as a normal cell phone call. The network then determines if the call is a one-to-one or a group call. If the call is a group call, the network provides the digital voice packets to each target in the group based on an address associated with each of the targets. A list containing addresses of targets may be sent to the network in a variety of different methods, including: using Short Data Burst (SDB) messages over Access Channel/Enhanced Access Channel (ACH/EACH), SDB messages over a Reverse Common Control Channel R-CCCH, in the origination message, and over the traffic channel itself. When the PTT button of the originating communication unit is pressed, the targets in the list are paged whereupon they may join the call by responding and this will be indicated with a Talk Permit Tone (TPT). Thus the setup delay for initiating the call is the time from PTT to the first TPT at the originating communication unit.
Problems can arise in that, in the typical slotted environment associated with, for example certain versions of Integrated Digital Enhanced Networks, communication units only wake up periodically, such as every 0.64, 1.28, 2.56, or 5.12 seconds to check for incoming pages including pages associated with the group call. Thus, significant delay may be incurred between when the first TPT is issued and when other targets join the call. Further problems involve audio clipping occurring while other targets are joining the group call after the first TPT. Thus, if setup delay is too long, audio delay and clipping will persist for a corresponding amount of time.
Several sources of delay are possible. For example, address transfer time for communication units is long, for example, from approximately 240 to 900 msecs to send 10 addresses, thus target communication units may go back to sleep during address transfer resulting in waiting an extra slot cycle and encountering more audio clipping. Further, if address data is sent across a traffic channel, an extra slot cycle of delay may result due to waiting for channel set up. If address data is sent prior to the traffic channel being built or setup, extra delay will result from a slot cycle time longer than a channel build time. It should be noted that in conventional group call set up, target addresses associated with address data are randomly ordered and prioritized during address transfer, causing an unnecessarily large number of targets to wait an extra cycle. The last address sent up, for example will have about a 70% chance of having to wait one additional cycle, or another 1.28 seconds, before getting a page associated with the group call.
Clearly a need exists for methods and apparatus capable of reducing the delay associated with a call set up.